Is your boat a DS I? with an open cockpit,
Sailors Tailor is one source for Day Sailer Cockpit covers, but any good canvas shop can make a cover if they measure the boat (they might already have a pattern anyway). SAILOR'S TAILOR also sells complete boat covers for the DS both for use on the trailer and in the water. They may have a good deal listed on the Day Sailer Class Association web site
www.daysailer.org
The DS II and DS III both have "self-bailing" cockpits and should not have a probelm being uncovered, I have a DS II and don't cover the boat on the mooring. Even the DS I can be left uncovered, but having at least a cockpit cover (boom tent) will keep out the rain and will keep the inside cleaner and help the varnish (or paint?) on the woodwork last longer.